Re: [PATCH] smaps: Fix the abnormal memory statistics obtained through /proc/pid/smaps

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Therefore, when obtaining pages through the follow_trans_huge_pmd
interface, add the FOLL_FORCE flag to count the pages corresponding to
PROTNONE to solve the above problem.


We really want to avoid the usage of FOLL_FORCE, and ideally limit it
to ptrace only.

Fundamentally when removing FOLL_NUMA we did already assumed !FORCE is
FOLL_NUMA.  It means to me after the removal it's not possible to say in a
gup walker that "it's not FORCEd, but I don't want to trigger NUMA but just
get the page".

Is that what we want?  Shall we document that in FOLL_FORCE if we intended
to enforce numa balancing as long as !FORCE?

That was the idea, yes. I could have sworn we had that at least in some patch description.

Back then, I played with special-casing on gup_can_follow_protnone() on FOLL_GET | FOLL_PIN. But it's all just best guesses.

Can always be added if deemed necessary and worth it.

Here, it's simply an abuse of that GUP function that I wasn't aware of -- otherwise I'd have removed that before hand.



Signed-off-by: liubo <liubo254@xxxxxxxxxx>
Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
---
  fs/proc/task_mmu.c | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index c1e6531cb02a..ed08f9b869e2 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -571,8 +571,10 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
         bool migration = false;

         if (pmd_present(*pmd)) {
-               /* FOLL_DUMP will return -EFAULT on huge zero page */
-               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+               /* FOLL_DUMP will return -EFAULT on huge zero page
+                * FOLL_FORCE follow a PROT_NONE mapped page
+                */
+               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP | FOLL_FORCE);
         } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
                 swp_entry_t entry = pmd_to_swp_entry(*pmd);

Might do as an easy fix. But we really should get rid of that
absolutely disgusting usage of follow_trans_huge_pmd().

We don't need 99% of what follow_trans_huge_pmd() does here.

Would the following also fix your issue?

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 507cd4e59d07..fc744964816e 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -587,8 +587,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
         bool migration = false;

         if (pmd_present(*pmd)) {
-               /* FOLL_DUMP will return -EFAULT on huge zero page */
-               page = follow_trans_huge_pmd(vma, addr, pmd, FOLL_DUMP);
+               page = vm_normal_page_pmd(vma, addr, *pmd);
         } else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
                 swp_entry_t entry = pmd_to_swp_entry(*pmd);

It also skips the shared zeropage and pmd_devmap(),

Otherwise, a simple pmd_page(*pmd) + is_huge_zero_pmd(*pmd) check will do, but I
suspect vm_normal_page_pmd() might be what we actually want to have here.

Because smaps_pte_entry() properly checks for vm_normal_page().

There're indeed some very trivial detail in vm_normal_page_pmd() that's
different, but maybe not so relevant.  E.g.,

	if (WARN_ON_ONCE(folio_ref_count(folio) <= 0))
		return -ENOMEM;

Note that we're not even passing FOLL_GET | FOLL_PIN. Because we're not actually doing GUP. So the refcount is not that relevant.


	if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
		return -EREMOTEIO;

I'm not sure whether the p2pdma page would matter in any form here.  E.g.,
whether it can be mapped privately.

Good point, but I don't think that people messing with GUP even imagined that we would call that function from a !GUP place.

This was wrong from the very start. If we're not in GUP, we shouldn't call GUP functions.

--
Cheers,

David / dhildenb




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